Synchrotron X-ray based characterization of technologically relevant III-V surfaces and nanostructures Troian, Andrea

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LUND UNIVERSITY

Synchrotron X-ray based characterization of technologically relevant III-V surfaces and nanostructures

Troian, Andrea

2019

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Troian, A. (2019). Synchrotron X-ray based characterization of technologically relevant III-V surfaces and

nanostructures. [Doctoral Thesis (monograph), Department of Physics]. Lund University , Department of physics.

Total number of authors:

1

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DREA TROIANSynchrotron X-ray based characterization of technologically relevant III-V surfaces and nanostructures 20

Faculty of Science Department of Physics Division of Synchrotron Radiation Research

Synchrotron X-ray based characterization of technologically relevant III-V surfaces and nanostructures

ANDREA TROIAN

DEPARTMENT OF PHYSICS | FACULTY OF SCIENCE | LUND UNIVERSITY

539780

Even Pollock would have loved doing cross-sectional STM!

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Synchrotron X-ray based characterization of technologically relevant III-V surfaces and nanostructures

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Synchrotron X-ray based

characterization of technologically relevant III-V surfaces and

nanostructures

Andrea Troian

DOCTORAL DISSERTATION

by due permission of the Faculty of Science, Lund University, Sweden.

To be defended in Lund, Lundmarksalen, Astronomihuset, Sölvegatan 27, on the 12th of April 2019 at 09:15.

Faculty opponent Prof. Andrew C. Kummel

University of California, San Diego, CA, USA.

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Organization LUND UNIVERSITY

Document name: DOCTORAL DISSERTATION

Date of disputation: 2019-04-12 Author(s): Andrea Troian Sponsoring organization

Title and subtitle: Synchrotron X-ray based characterization of technologically relevant III-V surfaces and nanostructures

Abstract

Innovative design and materials are needed to satisfy the demand for efficient and scalable devices for electronic and opto-electronic applications, such as transistors, LEDs, and solar cells. Nanostructured III-V semiconductors are an appealing solution, combining the excellent functional properties of III-V materials with the flexibility typical of nanostructures, such as the nanowires (NWs) studied here. However, there are a number of open challenges, that currently hinder the performance of III-V nanostructure devices: first, the surface quality of III-V materials is still one of their main limiting factors. Other problems specific of III-V NWs are the control of dopant incorporation - crucial for their functionalization -, and of their structural inhomogeneity (e.g. lattice strain and tilt), that can affect opto-electronic performance. These problematics require a set of non-trivial cutting-edge characterization tools:

here an approach based on a combination of X-ray synchrotron techniques is demonstrated.

Synchrotron based X-ray photoelectron spectroscopy (XPS) has been used to study the surface chemistry of III-V model systems and to monitor industrially relevant processing on them. A new passivation process improving the surface quality of InAs substrates used for electronics has been investigated: the surface structure and composition resulting from thermal oxidation followed by ex situ deposition of a high-k material via atomic layer deposition (ALD) has been assessed with XPS. The implementation of this passivation approach in gate stacks showed improvements in performance, that were attributed to the specific stoichiometry of the thermal oxide. The dynamics of the ALD process on InAs was also studied in situ with ambient pressure XPS: it was observed that the chemisorption of the precursor is an important step to ensure a good quality of the high-k oxide deposition.

Dopant evaluation in NWs is challenging due to their small dimensions. Here, a first approach to this problem was to perform XPS to study the effects of Zn dopant incorporation on the surface of GaAs NWs, used for solar cells.

High doping conditions during growth were found to form a Zn layer on the outside of the NW that suppresses the native oxides, which are generally a cause of poor passivation of III-V surfaces. In another experiment, XPS scanning microscopy was used to study surface Zn doping in an InP NW with an axial pn junction, also used for solar cells. The surface potential drop along the junction was monitored in operando, while applying a bias to the NW device, and it was found smaller than what expected for the bulk. Finally, a quantitative evaluation of Zn dopants incorporation in III-V NWs was studied for the first time with nano-focused X-ray fluorescence, due to the excellent combination of low detection limits and spatial resolution. Dopant gradients and memory effects were noted along InP and InGaP NWs, showing complex dopant incorporation mechanisms during the growth.

The structural inhomogeneity in InGaN nano-pyramids for next generation LEDs was also investigated. The influence of different processing parameters on lattice and strain were studied with full field X-ray diffraction microscopy. This imaging technique uses Bragg diffraction intensity as contrast mechanism and has a large field of view, useful for imaging at once large areas patterned with pyramids, giving valuable statistical consistency. The growth parameters providing the best lattice quality and homogeneity were assessed.

This thesis shows how cutting edge synchrotron characterization methods can provide useful information for improving III-V surfaces and nanostructures for next generation devices. Moreover, in most cases advances in the characterization methods are achieved, that can be relevant also in other and broader scientific fields.

Key words: synchrotron radiation, III-V semiconductors, high-k oxides, passivation, doping, XPS, AP-XPS, SPEM, XRF, Full field X-ray diffraction microscopy

Classification system and/or index terms (if any)

Supplementary bibliographical information Language English

ISSN and key title ISBN (print): 978-91-7753-978-0

ISBN (pdf): 978-91-7753-979-7

Recipient’s notes Number of pages 131 Price

Security classification

I, the undersigned, being the copyright owner of the abstract of the above-mentioned dissertation, hereby grant to all reference sources permission to publish and disseminate the abstract of the above-mentioned dissertation.

Signature Date 2019-03-04

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Synchrotron X-ray based

characterization of technologically relevant III-V surfaces and

nanostructures

Andrea Troian

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Front cover: false color map of an array of InGaN nanopyramids, measured with full field X-ray diffraction microscopy.

Back cover: optical microscope image of a sample holder for cross-sectional STM with a cleaved GaAs wafer sample.

Copyright pp. i-xxii, 1-109, front and back covers: Andrea Troian

Paper 1 © Reproduced from J. of Appl. Physics, with the permission of AIP Publishing. All rights reserved.

Paper 2 © published by AIP Publishing, CC-BY Paper 3 © published by Springer Nature, CC-BY Paper 4 © by the authors (manuscript unpublished)

Paper 5 © ACS Nano (ACS AuthorChoice Open Access License) Paper 6 © Cambridge University Press. Reproduced with permission.

All rights reserved.

Paper 7 © by the authors (manuscript unpublished) Division of Synchrotron Radiation Research Department of Physics, Faculty of Science Lund University

SE-221 00, Lund Sweden

ISBN 978-91-7753-978-0 (Print) 978-91-7753-979-7 (Pdf)

Printed in Sweden by Media-Tryck, Lund University Lund 2019

Media-Tryck is an environmentally certified and ISO 14001 certified provider of printed material.

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Table of Contents

Abstract ... ix

Populärvetenskaplig sammanfattning ... xi

Popular science summary ... xiii

List of papers ... xv

List of recurrent acronyms and symbols ... xviii

Acknowledgements ... xx

1. Introduction ... 1

1.1. Motivation ... 1

1.2. Outline of the thesis... 3

2. III-V nanostructures for new devices ... 5

2.1. Properties of III-V semiconductors ... 5

2.1.1 Semiconductor surfaces ... 7

2.2. From bulk to nanowires ... 8

2.2.1. Nanowire growth ... 10

2.3. Electronic and optoelectronic devices based on III-V semiconductors 15 2.3.1. Light emitting diodes ... 15

2.3.2. Solar cells ... 18

2.3.3. Metal oxide field effect transistors ... 19

2.4. III-V surfaces and their influence on device performance ... 21

2.4.1. Surface cleaning and passivation ... 22

2.4.2. Atomic layer deposition of high permittivity (high-κ) oxides ... 23

2.4.3. Thermal oxidation prior to ALD: a novel approach ... 26

3. X-rays: a suitable probe for surface and nanostructure characterization .. 27

3.1. Interaction of X-rays with matter ... 27

3.2. X-rays from synchrotron radiation sources ... 29

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4. X-ray photoemission spectroscopy ... 31

4.1. XPS: theoretical and practical aspects... 32

4.1.1. Quantum mechanical description of the photoelectric effect .... 33

4.1.2. Line shape and other features of the XPS spectra ... 35

4.1.3. Core level shifts ... 37

4.1.4. Probing depth of XPS ... 38

4.1.5. XPS experimental setup... 40

4.2. Synchrotron based XPS as a tool for investigating technologically relevant III-V surfaces ... 41

4.2.1. XPS study of effects of Zn doping on the surface of GaAs aerotaxy NWs ... 41

4.2.2. XPS surface study after subsequent processing steps: a new passivation treatment on InAs ... 43

4.3. Ambient pressure XPS ... 46

4.3.1. AP-XPS for studying in situ ALD on InAs ... 48

4.4. Scanning Photoelectron Microscopy ... 52

4.4.1. Equipment and operation modes ... 52

4.4.2. SPEM as a tool for operando surface potential measurements of solar cell NWs ... 54

5. Nanofocused X-ray fluorescence ... 59

5.1. Quantification in X-ray fluorescence ... 59

5.2. Nanofocused X-ray fluorescence: a tool for doping assessment in nanowires ... 66

6. Full Field X-ray Diffraction Microscopy ... 71

6.1. Basics of X-ray diffraction ... 71

6.2. Full field X-ray diffraction microscopy: principle and advantages ... 73

6.3. FFXDM on InxGa1-xN pyramids for LEDs ... 76

6.3.1 InxGa1-xN LEDs: opportunities and open problems ... 76

6.3.2 FFXDM study of InxGa1-xN pyramids ... 77

7. Complementary techniques ... 85

7.1 Scanning Tunneling Microscopy ... 85

7.2 Low Energy Electron Diffraction ... 88

8. Concluding remarks and future developments ... 91

References ... 95

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Abstract

Innovative design and materials are needed to satisfy the demand for efficient and scalable devices for electronic and opto-electronic applications, such as transistors, LEDs, and solar cells. Nanostructured III-V semiconductors are an appealing solution, combining the excellent functional properties of III-V materials with the flexibility typical of nanostructures, such as the nanowires (NWs) studied here.

However, there are a number of open challenges, that currently hinder the performance of III-V nanostructure devices: first, the surface quality of III-V materials is still one of their main limiting factors. Other problems specific of III-V NWs are the control of dopant incorporation - crucial for their functionalization -, and of their structural inhomogeneity (e.g. lattice strain and tilt), that can affect opto- electronic performance. These problematics require a set of non-trivial cutting-edge characterization tools: here an approach based on a combination of X-ray synchrotron techniques is demonstrated.

Synchrotron based X-ray photoelectron spectroscopy (XPS) has been used to study the surface chemistry of III-V model systems and to monitor industrially relevant processing on them. A new passivation process improving the surface quality of InAs substrates used for electronics has been investigated: the surface structure and composition resulting from thermal oxidation followed by ex situ deposition of a high-k material via atomic layer deposition (ALD) has been assessed with XPS. The implementation of this passivation approach in gate stacks showed improvements in performance, that were attributed to the specific stoichiometry of the thermal oxide. The dynamics of the ALD process on InAs was also studied in situ with ambient pressure XPS: it was observed that the chemisorption of the precursor is an important step to ensure a good quality of the high-k oxide deposition.

Dopant evaluation in NWs is challenging due to their small dimensions. Here, a first approach to this problem was to perform XPS to study the effects of Zn dopant incorporation on the surface of GaAs NWs, used for solar cells. High doping conditions during growth were found to form a Zn layer on the outside of the NW that suppresses the native oxides, which are generally a cause of poor passivation of III-V surfaces. In another experiment, XPS scanning microscopy was used to study surface Zn doping in an InP NW with an axial pn junction, also used for solar cells.

The surface potential drop along the junction was monitored in operando, while applying a bias to the NW device, and it was found smaller than what expected for the bulk. Finally, a quantitative evaluation of Zn dopants incorporation in III-V NWs was studied for the first time with nano-focused X-ray fluorescence, due to the excellent combination of low detection limits and spatial resolution. Dopant gradients and memory effects were noted along InP and InGaP NWs, showing complex dopant incorporation mechanisms during the growth.

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The structural inhomogeneity in InGaN nano-pyramids for next generation LEDs was also investigated. The influence of different processing parameters on lattice and strain were studied with full field X-ray diffraction microscopy. This imaging technique uses Bragg diffraction intensity as contrast mechanism and has a large field of view, useful for imaging at once large areas patterned with pyramids, giving valuable statistical consistency. The growth parameters providing the best lattice quality and homogeneity were assessed.

This thesis shows how cutting edge synchrotron characterization methods can provide useful information for improving III-V surfaces and nanostructures for next generation devices. Moreover, in most cases advances in the characterization methods are achieved, that can be relevant also in other and broader scientific fields.

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Populärvetenskaplig sammanfattning

En studie publicerad av NASA:s Goddard Institute for Space Studies år 2019 visade att de genomsnittliga globala temperaturerna under de senaste fyra åren (2015-2018) var de högsta som någonsin registrerats i människans historia. Den nyheten passar väl in i de alarmerande prognoserna om global uppvärmning, och kräver snabba och radikala handlingar. Att studera “tekniskt relevanta III-V halvledares nanostrukturer och ytor med hjälp av synkrotronljus”, som är en omformulering av denna avhandlings titel, syftar till att ge ett litet bidrag till denna fråga. Det är dock svårt att se ett enkelt samband mellan titeln och den globala uppvärmningen. Att klargöra avhandlingens beståndsdelar kan kanske hjälpa till att belysa kopplingen.

Halvledare, som t.ex. kisel (Si), är material som finns närvarande i hela vårat vardagsliv. De har två typer av laddningar som rör sig inuti: elektroner (negativa) och "hål" (positiva). Genom att lägga till orenheter som kallas "dopants" i halvledare, kan man få "n-dopning" när de flesta av laddningar är elektroner, eller

"p-dopning" när de är hål, beroende på dopant. p och n dopade halvledare kan kombineras för att skapa dioder och transistorer, som ofta används i datorer.

Dessutom kan, i lysdioder (mer kända som LEDs), hål och elektroner återförenas och generera fotoner, som producerar ljus. Halvledare är också avgörande för att fånga upp solenergi: fotoner kan generera ström och denna effekt utnyttjas i solceller.

Framsteg inom halvledarteknik kan ha stor inverka på miljön: genom att öka prestandan hos apparaterna kan energiförbrukningen minskas markant, samtidigt som effektivare solceller kan ge renare energi.

I den här avhandlingen studeras prover av klass III-V halvledare som är nära att bli tillämpningsbara. III-V:er är föreningar av två (eller flera) element, en som tillhör III-gruppen (t.ex. In, Ga, och Al) och en av V-gruppen (t.ex. N, As, och P) av den periodiska tabellen. III-V halvledare har enastående ledningsförmåga och effektiva egenskaper för att generera ljus. Detta gör dem till perfekta kandidater för framtida enheter: till exempel är indiumarsenid (InAs) optimalt för transistorer, indiumfosfat (InP) för solceller och indiumgalliumnitrid (InGaN) för LEDs.

III-V:er har faktiskt varit kända under lång tid och undersökts redan före Si.

Anledningen till att de inte har ersatt Si beror främst på den relativt höga kostnaden och den dåliga kvaliteten på deras ytor, vilket är avgörande för elektronik och solceller. Under de senaste decennierna, har forskning om dessa material upplevt en renässans, på grund av nya metoder för att förbättra ytorna och också för möjligheten att tillverka III-V halvledare som nanostrukturer, som t.ex. nanotrådar (NW) som studeras i denna avhandling. NWs är nålformade objekt som är cirka en mikrometer (1 μm = 0,000001 m) långa och med diametrar på cirka 30-200 nanometer (1 nm = 0,001 μm). De erbjuder en mycket flexibel grund för att

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kombinera olika III-V material utan att producera stora deformationsfält (som kan hämma prestandan) i samma nanostruktur. Många fysiska processer, som laddningstransport, är i sig effektivare i NWs än i materialets normala storlek.

För att utveckla banbrytande effektiva nanostrukturenheter behövs dock en fullständig förståelse av fysiken bakom dem. Framsteg måste uppnås i deras behandling genom att 1) förbättra ytkvaliteten, 2) kontrollera NW-dopningen, 3) övervaka den strukturella deformationen.

Karaktärisering av III-V ytor och NWs är därför nödvändigt, men detta är extremt utmanande på grund av deras ringa storlekar. Avhandlingen föreslår ett steg framåt i denna riktning genom att använda en kombination av avancerade tekniker baserade på röntgenljus från en synkrotronkälla. Röntgenljus som produceras av en synkrotron - en cyklisk partikelaccelerator - är optimala på grund av dess mycket höga intensitet, som är utmärkt för detaljerade kemiska och strukturella analyser och även mikroskopi, när ljusstrålen är ordentligt fokuserad.

En ny behandling för att förbättra kvaliteten på InAs-ytor undersöktes med en teknik som kallas fotoelektronspektroskopi (XPS). Denna metod är baserad på insamling av provelektroner som matas ut av röntgenljuset, som fungerar som kemiska fingeravtryck. Genom att utveckla denna teknik, studerades den ytkemiska utvecklingen under depositionen av ett högkvalitativt oxidskikt i realtid. Resultaten visade de viktiga aspekterna i denna behandling. XPS användes också för att studera dopants på NWs ytor, som visade sig ha en stark effekt på ytan egenskaper.

Resultatet av dopning i NWs är generellt svårt att förutsäga och mäta. För första gången har en röntgenstråle med ett fokus på bara några tiotals nanometer använts för att kartlägga dopants-distribution i III-V NWs. Dopants räknades tack vare deras emission av karakteristiskt röntgenljus, en effekt som kallas fluorescens. Resultaten är användbara för att förstå hur man optimerar tillverkningen för att få den önskade dopants-distributionen för effektiva NW-solceller.

Slutligen studerades de deformationsseffekter som orsakas av tillverkning av NWs- mönster som används för LEDs med en teknik som kallas full field röntgens- diffraktionsmikroskopi. Denna metod är baserad på diffraktion, en reflektion av röntgenstrålar som endast förekommer i specifika riktningar, beroende på atomavståndet, som självt beror på deformation. För första gången erhölls bilder på InGaN NW-mönster med kontrasten som ges av deras olika deformationer. Denna studie hjälper till att hitta de bästa parametrarna för att tillverka högkvalitativa NW LEDs.

Denna avhandling utforskar ett brett spektrum av viktiga aspekter i specifika III-V nanostrukturer. Användningen av moderna röntgenkarakteriseringsverktyg syftar till att ge konkreta svar för att förbättra dessa material och enheter, med förhoppningen att göra dem mer effektiva.

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Popular science summary

A study published by the Goddard Institute for Space Studies of NASA in 2019 put in evidence that the average global temperatures of the last four years (2015-2018) were the highest ever recorded in human history. This recent news fits well into the alarming forecasts on global warming, demanding prompt and radical interventions.

Studying “technologically relevant III-V semiconductor nanostructures and surfaces with techniques based on synchrotron radiation”, which is a rephrasing of the title of this thesis, aims to give a little contribution to this issue. Apparently, there is not a straightforward connection between the title and the global warming:

a clarification on the elements of this thesis can help in highlighting the link.

Semiconductors, like for instance silicon (Si), are materials present in almost all aspects of our everyday life. They have two different types of charges that can move in the solid: electrons (negative) and “holes” (positive). By adding impurities called

“dopants” into semiconductors, one can obtain “n type doping” when most of the charge carriers are electrons, or “p type doping” when they are holes, depending on the dopant. p and n doped semiconductors can be combined together to create diodes and transistors, widely used in computers. Moreover, in the class of the light emitting diodes, better known as LEDs, holes and electrons can recombine together generating photons, that is producing light. Semiconductors are also crucial in harvesting solar energy: the photons can generate a current and this effect is exploited in solar cells.

Advances in semiconductor technology can have a big impact for the environment:

by boosting the performance of devices, the energy consumption can be sensibly decreased, while more efficient solar cells can provide more clean energy.

Here, samples of the class of the III-V semiconductors close to realistic applications are studied. III-Vs are compounds of two (or more) elements, one belonging to the III group (for example In, Ga, and Al) and one of the V group (for example N, As, and P) of the periodic table. III-V semiconductors have outstanding charge transport and charge-photon conversion properties, making them perfect candidates for future devices: for example, indium arsenide (InAs) is optimal for transistors, indium phosphate (InP) for solar cells and indium gallium nitride (InGaN) for LEDs.

III-Vs have actually been known for a long time and investigated even before Si.

The reason why they have not supplanted Si is mainly due to the relatively high cost and to the poor quality of their surfaces, which is crucial for electronics and solar cells. In the last decades, the research on these materials experienced a renaissance, due to new methods for improving the surfaces and especially for the possibility of implementing III-V semiconductors in nanostructures, like for instance the nanowires (NW) studied in this thesis. NWs are needle shaped objects ca. 1 micron (1 µm = 0.000001 m) long and with diameters of ca. 30-200 nanometers (1 nm =

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0.001 µm), and they offer a very flexible platform to combine different III-V materials avoiding large strain fields (that can hamper performances) in the same nanostructure. Many physical processes, like charge transport, are intrinsically more efficient in NWs than in their bulk counterpart.

However, for developing cutting-edge efficient nanostructure devices, a complete understanding of the physics behind them is still needed. Advances need to be achieved in their processing by 1) improving the surface quality, 2) controlling the NW doping, 3) monitoring the structural strain.

A characterization of III-V surfaces and NWs is therefore needed, but this is very challenging due to their small size. This thesis proposes a step forward in this direction by using a combination of advanced techniques based on X-rays from a synchrotron source. The X-rays produced by a synchrotron - a cyclic particle accelerator - are ideal because of their very high intensity, excellent for detailed chemical and structural analyses and even microscopy, when focused properly.

A new processing to improve the quality of InAs surfaces was investigated with a technique called synchrotron based X-ray photoelectron spectroscopy (XPS). This method is based on the collection of the sample electrons ejected by the X-rays, acting as chemical fingerprint. By pushing this technique to its limits, the surface chemistry evolution during the deposition of a high quality oxide layer was studied in real time. The results showed the critical aspects in this industrially relevant processing. XPS was also used for studying the dopants on the surface of NWs, that were found to have a strong effect on the surface properties.

The doping incorporation in NWs is in general difficult to predict and measure. For the first time, an X-ray beam with a focal spot of only few tens of nanometers was used to map the dopant distribution in III-V NWs. The dopants were identified and counted thanks to their emission of characteristic X-rays, an effect called fluorescence. The results are useful to understand how to tailor the processing to have the desired dopant distribution for very efficient NW solar cells.

Finally, the strain effects caused by patterning arrays of NWs used for LEDs were studied with a technique called full field X-ray diffraction microscopy. This method is based on diffraction, a selective reflection of X-rays that occurs only in specific directions, depending on the atomic spacing: if there is strain, the atomic spacing (and the diffraction angle) is different. For the first time, images of InGaN NW arrays were obtained with the contrast given by their different strain. This study helps to find the optimal parameters for fabricating high quality NW LEDs.

This thesis explores a wide range of criticalities in specific III-V nanostructures.

The use of modern X-ray based characterization tools is aimed to give concrete answers to improve these materials and devices, with the hope of making them more energetically efficient.

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List of papers

This doctoral thesis is based on the following papers, which will be referred to in the text by their Roman numerals.

Paper I

Surface smoothing and native oxide suppression on Zn doped aerotaxy GaAs nanowires

S. Yngman, S. R. McKibbin, J. V. Knutsson, A. Troian, F. Yang, M. H. Magnusson, L. Samuelson, R. Timm and A. Mikkelsen

Journal of Applied Physics 2019, 125 (2), 025303.

I took part in the XPS experiment and contributed to the discussion of the manuscript.

Paper II

InAs-oxide interface composition and stability upon thermal oxidation and high-k atomic layer deposition

A. Troian, J. V. Knutsson, S. R. McKibbin, S. Yngman, A. S. Babadi, L.-E.

Wernersson, A. Mikkelsen and R. Timm AIP Advances 2018, 8 (12), 125227

I took part in the experiment planning and in the XPS, STM, and LEED experiment, I did the data analysis and I was the main responsible for writing the manuscript.

Paper III

Self-cleaning and surface chemical reactions during hafnium dioxide atomic layer deposition on indium arsenide

R. Timm, A. R. Head, S. Yngman, J. V. Knutsson, M. Hjort, S. R. McKibbin, A.

Troian, O. Persson, S. Urpelainen, J. Knudsen, J. Schnadt and A. Mikkelsen Nature Communications 2018, 9 (1), 1412.

I took part in the AP-XPS experiment and I contributed to the discussion of the manuscript.

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Paper IV

Operando surface characterization of an InP p-n junction nanowire diode S. R. McKibbin, J. V. Knutsson, J. Colvin, A. Troian, J. Webb, G. Otnes, K.

Dirscherl, H. Sezen, M. Amati, L. Gregoratti, M. Borgström, A. Mikkelsen and R. Timm

In manuscript

I took part in the experiment planning and in the SPEM experiment, and I contributed to the discussion of the manuscript.

Paper V

Nanobeam X-ray Fluorescence Dopant Mapping Reveals Dynamics of in Situ Zn-Doping in Nanowires

A. Troian, G. Otnes, X. Zeng, L. Chayanun, V. Dagytė, S. Hammarberg, D.

Salomon, R. Timm, A. Mikkelsen, M. T. Borgström and J. Wallentin Nano Letters 2018, 18 (10), 6461-6468.

Selected for “ESRF highlights 2018”

I took part in the experiment planning and in the nano-XRF experiment, I did the nano-XRF data analysis and I was the main responsible for writing the manuscript.

Paper VI

Lattice Tilt Mapping using Full Field Diffraction X-Ray Microscopy at ID01 ESRF

T. Zhou, T. Stankevic, A. Troian, Z. Ren, Z. Bi, J. Ohlsson, L. Samuelson, J.

Hilhorst, T. Schulli, A. Mikkelsen and O. Balmes Microscopy and Microanalysis 2018, 24 (S2), 126-127

I took part in the experiment planning and in the FFXDM experiment and I contributed to the manuscript discussion.

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Paper VII

Full field X-ray diffraction imaging of structural variations in arrays of InGaN nanopyramids

A. Troian, Z. Ren, T. Zhou, T. Stankevic, R. Timm, Z. Bi, J. Ohlsson, L.

Samuelson, J. Hilhorst, T. Schulli, O. Balmes, and A. Mikkelsen In manuscript

I took part in the experiment planning and in the FFXDM experiment, I did the FFXDM data analysis and I was the main responsible for the manuscript writing.

Publications to which I contributed that are not included in this dissertation:

Paper VIII

A. Troian, L. Rebuffi, M. Leoni, P. Scardi, Toward a reference material for line profile analysis. Powder Diffraction 2015, 30 (S1), S47-S51.

Paper IX

L. Rebuffi, A. Troian, R. Ciancio, E. Carlino, A. Amimi, A. Leonardi, P. Scardi, On the reliability of powder diffraction Line Profile Analysis of plastically deformed nanocrystalline systems. Scientific Reports 2016, 6, 20712

Paper X

L. Chayanun, V. Dagytė, A. Troian, D. Salomon, M. T. Borgström, J. Wallentin, Spectrally resolved x-ray beam induced current in a single InGaP nanowire.

Nanotechnology 2018, 29 (45), 454001.

Paper XI

L. Chayanun, G. Otnes, A. Troian, S. Hammarberg, D. Salomon, M. T. Borgstrom, J. Wallentin, Nanoscale mapping of carrier collection in single nanowire solar cells using X-ray beam induced current. Journal of Synchrotron Radiation 2019, 26 (1)

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List of recurrent acronyms and symbols

ALD Atomic layer deposition

AP-XPS Ambient pressure X-ray photoemission spectroscopy

CB Condution band

CDI Coherent diffraction imaging CRL Compound refractive lens

Eb Binding energy

EDS Energy dispersive X-ray spectroscopy EEA Electron energy analyzer

Ef Fermi level

Ek Kinetic energy

FFXDM Full field X-ray diffraction microscopy

FoV Field of view

FZP Fresnel zone plate

HSB Hue-saturation-brightness IMFP Inelastic mean free path LED Light emitting diode

LEED Low energy electron diffraction MCP Micro channel plate

MOSFET Metal oxide semiconductor field effect transistor MOVPE Metal organic vapor phase epitaxy

nano-XRF Nanofocused X-ray fluorescence microscopy

NW Nanowire

PEEM Photoemission electron microscopy QCSE Quantum confined Stark effect SAE Selected area epitaxy

SPEM Scanning photoemission microscopy/microscope SPV Surface photovoltage

STM Scanning tunneling microscopy/microscope TDMA-Hf Tetrakis-dimethylamino hafnium

TMA trimethylaluminum

UHV Ultra high vacuum

VB Valence band

Wz Wurtzite

XPS X-ray photoemission spectroscopy

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XRD X-ray diffraction

XRF X-ray fluorescence

Zb Zinc blende

κ Permittivity (also referred as k)

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Acknowledgements

A thesis usually has the name of only one author on the front page: this does not entirely reflect the truth, since a thesis is the result of a long lasting team work, that would not have ever seen the light without the contribution of many different people.

And making an ordered list of people to be thanked is tough, you are so many and I thank you all! I tried to keep some order in this list, but then it got out of my hands and got unorganized, a bit like my office desk: I apologize if I forgot someone.

I would start thanking the coach of this team work, my supervisor Rainer Timm, for always supporting and motivating me during my studies and always finding time for me: I am quite sure that this is one of the most appreciable supervisor’s virtues from a PhD student perspective. It was really a pleasure working with you, and I learnt an amazing quantity of stuff just from our discussion, from your feedbacks or just when sitting at a beam line doing experiments.

I am also very indebted towards my co-supervisor, Anders Mikkelsen: I am very grateful for introducing me to many different fields of synchrotron and STM science and to involve me in many different and exciting projects. It was great to find a co- supervisor with the door always open for me and with a such positive attitude, that always boosted my morale even in the toughest experiments.

Thank you both, I have been very lucky in having you as supervisors!

A special acknowledgement goes to Jesper Wallentin: working with you on the nano-XRF project has been an awesome opportunity, with so much science and super-exciting beam times. I have learnt so many different things that I consider the nano-XRF project as a PhD inside the PhD. I would like to also thank Magnus Börgström, Gaute Otnes, Xulu Zeng and Vilgailė Dagytė from FTF for the excellent collaboration in this project.

I am very indebted to Olivier Balmes for introducing and guiding me into a new dimension of XRD: it was real fun to work at the nano-pyramids imaging project.

Special thanks goes to Zhe Ren, one of the kindest and most XRD-expert colleagues I’ve ever met. I would also like to thank Thao Zhou, one of the most science- enthusiast colleagues I have ever worked together, with wonderful ideas and endless energy. I am also very thankful to Zhaoxia Bi and Olof Hultin for growing the samples and for the fruitful discussions.

I am also very grateful to Lars Erik Wernersson, for all the discussions about our nano-electronics projects together. I would like to acknowledge also Aein S. Babadi, for the precious collaboration on the project about thermal oxidation of InAs.

Doing experimental science is though, disappointing sometimes: it would be even tougher if during beam times (and STM-times) I wouldn’t have had wonderful

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colleagues like Sofie Yngman, Lert Chayanun, Sarah McKibbin, Zhihua Yong, Yen-Po Liu, Yi Liu, Susanna Hammarberg and Dimitry Dzhigaev.

Beam times would not be possible if there were not expert and helpful beam line scientists like Damien Salomon and Matteo Amati: thank you for all your support!

I would like to thank Heiner Linke and Gerda Rentschel for organizing the PhD4Energy project. One of the nicest things about PhD4Energy was doing the internship: I would like to thank Dina Carbone, Virginie Chamard and Marc Allain for this wonderful opportunity!

I would like to thank the two administrative pillars of our division that makes our life much (much) easier: Anne Petterson-Jungbeck and Patrik Wirgin. Patrik, be aware I will ask you one more time whether I have to put the tick on “diary allowance” or not.

A great (great) thank you goes to Lisa Rullik, for being an excellent friend, colleague and proof-reader and for always supporting me. Talking about great proof readers, I am very indebted to Hanna Dierks: thank you for your support during the thesis writing! A special thanks goes to Bart Oostenrijk, for sharing with me not only super-precious help in programming, but also a sharp sense of humor and the passion for bicycles. Talking about passions, I would like to thank Lukas Wittenbecher (and Veronika!) for all the support and for our board game nights! I am very grateful to Stefano Albertin and Giulio D’Acunto, who with all the laughs, talks, and dinners made me always feel at home (and we got an espresso machine).

Since Payam Shayesteh is an expert of the motto “daje Roma”, I would like to thank him here among the Italians, for all the talks, laugh and support during my thesis writing. When it comes to languages: tusen tack till Susanna och Sofie för att hjälpa mig varje dag att förbättra min Svenska!

I would like to thank Erik Malm for being an excellent office mate and Jovana Colvin for sharing with me the joys and pains of lab supervision. I would like to thank Sandra Benter, Yi Liu, Tamires Gallo, Foqia Rehman, Virginia Boix, Veronica Linpé, Claudiu Bulbucan (ciao, ragazzo!), Edvin Lundgren, Estephania Lira and all my colleagues of SLJUS for all the scientific and non-scientific discussions and for making SLJUS a very nice and enjoyable working place.

I gratefully remember also my previous co-workers at SLJUS: I would like to start thanking Niclas Johansson, a very good friend and probably the most Igor expert person I have (and will ever) met. I would like to thank Martin Hjort, Olof Persson and Johan Knutsson for initiating me to the cult of STM and STS, Milena Moreira for sharing beam times and the passion for art, Elin Grånäs and Olesia Snezhkova for being excellent office mates and for our coffee breaks, and Andreas Schaefer for all your energy, good tips and friendship. I would like to thank Mattias Åstrand, for being a great STM colleague and a dear friend sharing my passion for videogames!

Having beam times at Elettra was for me more fun than any other beam time, also

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because of the nice discussions and lunches with Luca Rebuffi: thank you for all your support during these years.

I would especially like to thank two colleagues that I have known from day -45, already at the interviews: Artis Svilans and Vilgailė Dagytė (it has been great working with you!).

I would like to thank all the friends here in Lund, especially Annita, David, Christel, Marie-Laure, Tasos, Lisa Z., Hadi, Orwah, and Alireza.

A special thanks goes to my dear friends Stefano R., Francesca, Francesco and Mathias: all far from me in distance, but always close, who gave me a huge support.

And how could I forget my dear friends Rocco and Stefano Z.?

Vorrei infine dedicare un grazie speciale alla mia famiglia: ai miei genitori ed alla mia sorella, ai miei nonni, ai miei zii ed alle mie cugine, ad Antonio, Giuliana, Paola ed a tutta la Lubiana crew, per avermi sempre sostenuto in tutti questi anni. Ed ovviamente, grazie Roberta: tutto questo (e non solo!) non sarebbe semplicemente stato possibile senza di te.

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1. Introduction

1.1. Motivation

In the last decades it has become more and more challenging to reduce the consumption of resources and at the same time satisfy an unstoppable growth of goods demand in the global market. In addition to this, global warming1 and pollution issues impose sustainable long term policies for energy harvesting and consumption, in which renewable energy sources will play the lion part in the future.

In order to tackle these challenges, a technological revolution is needed, especially into the semiconductor industry. The ambitious aim is to research and produce efficient electronic and optoelectronic components for mass production, like transistors, solar cells and lighting devices. More specifically, the goal is to look for new materials and innovative design architectures: an interesting approach is to shape these new materials into nanostructures, for compact and efficient devices.

Regarding the research on new semiconductor materials, nowadays almost the totality of electronics is based on silicon (Si), which is relatively inexpensive and easy to manufacture and functionalize in devices. However, Si does not have the best functional properties per se2, and its indirect band gap in the infrared spectral range puts fundamental limits to performance in opto-electronic applications, like e.g. solar cells. Moreover, Si based devices are approaching their scalability limits and new substitutes of Si are needed for small and more efficient devices.

For these reasons, one of the most effective approaches to develop new efficient and powerful devices is to find novel material candidates to replace Si: considering the huge market share of electronics, even marginal improvements in performance gained from substituting Si can have a big impact in energy consumption (and also in energy harvesting, in the case of photovoltaic cells).

One promising candidate for these applications is constituted by the class of III-V semiconductors. They are compound materials which combine one or more elements of group III (e.g. Al, Ga, and In) with elements of group V (e.g. N, P, As, and Sb) of the periodic table. In general, these materials are characterized by a high charge carrier mobility3-4 and a direct band gap, that can be tuned by alloying different III and V elements. These factors determined the interest in performing research on these materials, which are actually already used successfully in industry and can be found in everyday devices.

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The possibility of implementing such III-V materials in nanostructures, such as the rod-shaped nanowires (NWs) that are discussed in this thesis, has furthermore expanded the possible device designs, opening new intriguing perspectives on scalability, functionality, and efficiency.

III-V NWs can be functionalized by designing heterostructures, i.e. combining different III-V materials in the same structure. Another milestone in NW functionalization is given by the possibility of controlling the doping, i.e. the incorporation of intentional impurities in the NW, which strongly affects the charge transport characteristics. Moreover, III-V NWs are highly viable for integration with the established Si technology5: the strain due to the different lattice constant of the Si substrate and the NW can be easily relaxed due to the small footprint of the NW.

This compatibility of III-V NWs with inexpensive Si substrate makes them very interesting candidates for industrial scale production.

However, a fundamental understanding of III-V semiconductor nanostructures is still needed. The complete knowledge of the underlying physics and chemistry can be usefully capitalized on novel applications which can have a strong impact in the urgent demands of society stated above.

In this thesis a step forward in this direction is presented: different characteristics relevant for device performance have been identified and selected, and the characterization of these properties has been addressed with a complementary variety of techniques. Several open problems are explored in this thesis: first, one of the biggest hindrances to the development of III-V technology is the poor surface quality, which can be improved with dedicated processing approaches, whose mechanisms and physical and chemical electronic effects on the surface are yet not completely understood. The open problem of surface quality and its intimate correlation with technologically relevant processing is of high interest, since it affects both flat surfaces and nanostructures. Regarding NWs, a new degree of control and understanding of doping and structural quality is needed. High precision in concentration and spatial distribution of dopants is needed to obtain the desired functionality, whereas structural defects, such as lattice strain and tilt can compromise opto-electronic features. While a more detailed discussion on the relevance of these aspects will be provided afterwards, it is important to underline here that a more complete control of these aspects of NWs can pave the way to a wider spread in their applications.

X-rays are an excellent probe to characterize the structure and chemistry of these systems, and they have been used in this thesis as the main investigation tool. A wide plethora of advanced characterization techniques involving different kinds of matter-radiation interactions is available: in this work, I will focus on X-ray photoelectron spectroscopy (XPS) and its variants, X-ray fluorescence (XRF) microscopy, and X-ray diffraction (XRD) imaging. Moreover, the work presented here relies on X-rays produced by synchrotron sources, that offer unprecedented

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possibilities in terms of high brilliance, coherence, and energy tunability, compared to traditional X-ray sources. Dedicated cutting edge X-ray optics also allows to perform microscopy studies with the aforementioned techniques, which is fundamental in case of nanostructures. Due to these advantages, synchrotron based techniques are an excellent resource to integrate and complete the information on the nanostructures obtained with standard laboratory techniques, such as scanning probe and scanning electron microscopy and optical and electrical measurements.

The viable information from synchrotron X-ray based tools has been extended, whenever possible, by performing in situ and operando studies. This approach brings fundamental studies a step closer to real applications: in situ studies allow to study a sample of interest, for instance a surface deposition treatment, in environmental conditions close to industrial standards. Operando studies instead are relevant when studying devices, since they can unveil physical phenomena during operation.

In most of the cases of study treated in this dissertation, the characterization approaches are not routinely used in academia and/or industry, but are new. This is actually one of the main motivation for this work, i.e. the development of new characterization approaches adequate for successfully investigating novel material system for future efficient devices.

1.2. Outline of the thesis

This thesis is structured in two main blocks: a first introductory part is meant to guide the reader to the second part, which is a collection of scientific articles and manuscripts. The goal of the introductory part is to give a general background on the topics covered in the whole thesis, contextualizing the papers of the second part in a congruent scientific framework and making the results accessible to a wider audience. The numbering order of the papers is intended to be coherent with the structure of the introductory part. Whenever needed, the reader is readdressed to specialized literature for the details.

In Chapter 2, a general background on the III-V semiconductor systems of interest is provided. The aim is to provide general information about III-V systems, their structure and surface properties, and their implementation in fundamental devices.

The chapter underlines the open scientific questions about the control of processing aspects which can undermine the real device performances.

Chapter 3 introduces a powerful and flexible approach to tackle these problematics, that is X-ray based characterization. This chapter provides a framework for the following ones, pointing out the X-ray interactions that can be experimentally

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exploited, explaining also the benefits of synchrotron X-rays compared to lab sources.

From Chapter 4 to 7, the experimental techniques that I used during my PhD project are discussed, with the aim of answering the open issues stated in the previous chapters. For this reason, the main experimental results are shown right after the respective technique. The detailed results, supported also by other characterization tools, are fully described in the papers, whereas this dissertation aims to pinpoint the practical relevance of the results in a wider context and to give a common framework for the papers.

Chapter 4 deals with III-V surface characterization with X-ray photoemission spectroscopy (XPS) and its variants. The theoretical and experimental grounds are discussed, introducing the results of Papers I and II: high resolution XPS was used for studying the effects of doping on NW surfaces (Paper I) and to monitor a surface processing on a III-V substrate (Paper II). A variant of XPS, the ambient pressure XPS, is then discussed since it has been used in Paper III. The highlights of this publication regarding an in situ investigation of a thin layer deposition on InAs are presented. Scanning photoemission microscopy is also treated, since it has been used in Paper IV to study the surface potential of NWs devices during operation.

Chapter 5 shows a novel characterization approach based on X-ray fluorescence microscopy for doping quantification in III-V nanowires; the chapter introduces paper V.

In Chapter 6 X-ray diffraction imaging, a powerful technique to study strain in nanostructured materials is introduced. The case of interest (discussed in detail in Paper VI and VII) regards NW (or better, “nano-pyramids”) arrays relevant for solid state lighting, in which the relation between structural inhomogeneity and manufacturing parameters is studied.

Chapter 7 briefly introduces complementary laboratory techniques which were needed for a well-rounded characterization of the samples, like scanning tunneling microscopy (STM) and low energy electron diffraction (LEED) used in Paper II.

In Chapter 8 concluding remarks are stated, stressing the relevance of the results in answering the questions motivating this project. Open questions and challenges are also stated and future possible developments are suggested.

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2. III-V nanostructures for new devices

2.1. Properties of III-V semiconductors

This thesis focuses on III-V semiconductors with different chemical composition and shape. Here, an overview on features that are common to the whole III-V family are presented to give the reader a proper framework for the specific material systems and cases of study described afterwards in the thesis.

The chemical bond in III-V semiconductors is sp3, which is mainly covalent, with a certain amount of ionic character6 due to the difference in electronegativity between the group III and V constituents. Most of the III-V materials (e.g. InP, InAs, GaAs, etc…) crystallize in the zinc blende (Zb) structure (Figure 2.1a, left), which can be regarded as the resultant of two face centered cubic sublattices of the single components, translated by a [¼ ¼ ¼] vector in atomic units. Depending on the ionic character of the bonds, some III-V materials (e.g. GaN) can also crystallize in the wurtzite (Wz) form (Figure 2.1b, left): similarly, Wz can be seen as two interpenetrated hexagonal lattices. Wz and Zb are actually morphologically similar since they differ only in the stacking sequence along the [111] direction (corresponding to the [0001] direction in the hexagonal lattice coordinates). This can be better visualized by considering bilayers of atomic planes, e.g. pairs of group III and group V planes: the stacking sequence is ABCABC… for Zb (Figure 2.1a, right), whereas it is ABAB… for Wz (Figure 2.1b, right).

In addition to binary components, optoelectronic properties like the band gap of III-V semiconductors can further be modulated by alloying two or more different types of group III and/or V components, giving rise to ternary or quaternary alloys.

In this thesis, two ternary alloys have been explored: InxGa1-xN nano-platelets and GaxIn1-xP nanowires, which will be described more in detail in the dedicated sections.

Conductivity of III-V semiconductors can be tailored by doping7, i.e. by the addition of atomic impurities, which increases the number of free charge carriers. Dopants typically belonging to group II of the periodic table (e.g. Zn) act as acceptors, i.e.

at temperatures (T) higher than 0 K, they get ionized by accepting electrons from the semiconductor valence band, increasing the number of holes there, which are

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the majority charge carriers. The chemical potential of the p doped material is different from the undoped (or intrinsic) case, and the Fermi level Ef,p is shifted towards the valence band. Dopants from group VI act instead as donors, i.e. the additional valence electrons can be excited to the conduction band (at T>0 K), increasing the number of electrons there, which become the majority charge carriers.

In this case, the Fermi level Ef,n of the n doped semiconductor is shifted towards the conduction band. When combining a sequence of p and n doped regions, the Fermi levels Ef,p and Ef,n are aligned at thermal equilibrium, causing band bending of the p and n sides, which act as a barrier for the further diffusion of free charge carriers to the other side. This barrier height, and therefore the resistivity of the pn junction can be controlled with a proper bias, and this concept is at the basis of most optoelectronic and electronic devices, as explained afterwards. A detailed discussion about the properties of pn junctions can be found in references7-8. Doping is crucial in controlling the functionality of semiconductor devices. This is even more important for nanostructured devices, where even small amounts of dopants can dominate their behavior. Controlled doping incorporation in nanostructures is therefore an active field of research, which requires adequate doping characterization tools9: parts of this thesis (Papers I, IV and V) are dedicated to investigate open issues related to doping in nanostructures.

Figure 2.1: III-V bulk lattices: zinc blende structure (a) is composed by two interpenetrating face centered cubic structures (left), and can be seen as stacking sequence of ABCABC III-V bilayers (a and b represent layers of III and V atoms). Wurtzite (b) is composed of two hexagonal closed pack structures (left) and can be seen as a sequence of ABAB bilayers. [Drawn with VESTA software10]

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2.1.1 Semiconductor surfaces

As in most material systems, surfaces play a major role also in semiconductor devices, since this is the region interfacing with the extern.

Surfaces constitute necessarily a disruption of the crystal symmetry, and the atoms at the boundary have under-coordinated (or dangling) bonds. This means that there are additionally available states localized at the surface, pointed out with the generic term surface states. Therefore, the chemical potential of even a perfect defect-free surface is in general different from the one of the bulk (Figure 2.2a). At thermal equilibrium, the chemical potential of bulk and surface is equalized11-12, implying the formation of a space charge region beneath the surface (Figure 2.2b). The space charge region can for example be caused by the filling of the unsaturated dangling bonds at the surface with free electrons. This charge accumulation at the surface is balanced by the ionized donors from the inner region. The formation of this electrostatic potential consequently causes a band bending towards the surface, and its entity can be modelled with Poisson’s equation.

Similar phenomena at the surface can also be described for more complicated (but more useful, in practice) interfaces, i.e. semiconductor-metal and semiconductor- insulator. In case of the semiconductor-metal interface, a spatial charge is formed at equilibrium to level up the respective Fermi levels. A band bending results from the charge exchange between the metal and the semiconductor. The entity and sign of this band bending (known as Schottky barrier) depends mainly on the work function of the metal and on electron affinity and doping of the semiconductor. It is noteworthy that this concept describes the ideal Schottky barrier, which depends only on intrinsic properties of the two separated systems.

Figure 2.2 a) On the left: the band energy diagram with the surface not at equilibrium, in the case of a n doped semiconductor. The Fermi level of the surface (Ef,surf) is not aligned with the Fermi level of the bulk (Ef). CB and VB are respectively the conduction and valence band edges. On the right: representation of atomic bonding: atomic filled bonds are represented as traits filled with dark pink, unfilled bonds as void traits. The pink area represents the electrons in CB. b) Surface at equilibrium: band diagram on the left, with Ef,surf = Ef and formation of the space charge layer (“+” signs). In the bond sketch on the right, the faded pink area represents the space charge region, depleted of electrons.

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In reality, one has to consider also the effects of surface states at the interfaces, which can modify the barrier properties.

The interface towards an insulator or oxide is, ideally, similar to the semiconductor- vacuum case, since no charge exchange can take place at the interface. However, additional polarization charges and impurities (e.g. interface traps or oxide charges) will influence the number of surface states. The impact of the surface states on device performance will be discussed more in detail in Section 2.4.

An important factor in determining the amount and type of surface states is actually the atomic plane cutting the crystal and defining the surface. Technologically relevant surface orientations11 are the (100), (111) and (110) facets, defined by their Miller indices (hkl). Actually, the atomic pattern defined by these facets (except the (110)) is often not preserved, since these conformations leave many dangling bonds, which are energetically not stable. The surface usually undergoes a reconstruction, which means that the surface atoms self-reassemble into a structure of lower energy.

These patterns, which are important in determining the electronic states of the surface can be determined by low energy electron diffraction and scanning tunneling microscopy, which will be discussed in Chapter 7.

2.2. From bulk to nanowires

Nanostructured materials have one or more dimensions in the nanometer range and their thermodynamics and functional properties13 may differ significantly from their bulk counterpart. Nanostructures can be classified depending on their characteristic dimensions, which determine the density of states and charge transport properties14; one can identify zero-dimensional nanostructures, such as quantum dots, one- dimensional, such as nanowires (NWs), and two-dimensional nanostructures, such as graphene and other thin films15. In this dissertation I focus on planar substrates, i.e. three-dimensional samples with a flat surface, and on NWs. Flat substrates are in fact an useful model systems and a simple case of study that can be extended to NWs16 in a second moment.

III-V NWs are needle shaped structures of III-V semiconductors with a high aspect ratio, in which the radius ranges typically between 10 and 200 nm and the length can extend up to several µm17 (Figure 2.3).

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Figure 2.3: III-V nanowires examples. a) InP NW array for solar cell applications [adapted from Paper V]. b) InAs NWs for electronics; Wz and Zb segments are put in evidence [courtesy of Dr. Sebastian Lehmann]. c) InxGa1-xN nano-pyramids for LED applications [courtesy of Dr. Zhaoxia Bi].

They are promising candidates for electronic and opto-electronic applications due to their high charge carrier mobility, the direct band gap, and the possibility of incorporating dopants in situ with high control. The remarkable freedom in designing defect-free heterostructures18-19 in NWs is also one of the major breakthroughs for their implementation in electronic devices. For example, axial heterostructures in III-V NWs are viable due the low radial dimensions. They in fact allow to accommodate the strain at the interface caused by the different lattice parameters and to compensate for different thermal expansion coefficients, assuring an epitaxial continuity of the two matching phases.

III-V nanowires can also be interfaced directly with Si substrates5, 20: the (hetero)epitaxial growth of III-V on Si is particularly attractive from a practical point of view, since the process can be potentially integrated in the existing Si technology platform. The feasibility of the concept has already been demonstrated and implemented in real opto-electronic devices21.

When it comes to devices, the high aspect ratio of NWs can result very useful in wave guiding effects for solar cells applications22 or for improved electrostatic control for electronics23, as it will be discussed afterwards. Interestingly, the limited radial dimensions in NWs allow also to obtain crystallographic variants that are not stable in the bulk form24: this feature can be interesting for quantum devices, in which a Wz quantum well can be for instance inserted between two Zb segments17. The high surface to bulk ratio of nanowires has however also the consequence that the electronic behavior can be dominated by the surface, affecting the whole nanostructure. An example is the Fermi level pinning effect, that can be summarized as the inability to change the semiconductor Fermi level due to a high number of surface states; this effect can extend to the whole NW diameter, dominating its behavior. For this reason, an effective passivationI of electronically active defects is even more important for nanostructures.

I Passivation will be discussed more in detail in Section 2.4. For now, it can be defined as processing reducing the active surface defect density.

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2.2.1. Nanowire growth

Nanowire growth is a broad research field and a wide variety of NWs with different composition, shape and properties can be produced: the goal of this section is therefore not to give a complete overview on the topic, but only to contextualize the samples presented in the papers.

Nowadays, a plethora of technologies is available for growing needled shaped structures such as NWs, which can be categorized in two big families: top-down and bottom-up approaches. In the top-down paradigm, a bulk material is consumed selectively giving rise to a NW pattern, whereas the bottom-up approach consists in growing NW structures by assembling them from basic building blocks25. All the NWs treated in this thesis are produced with a bottom-up approach, which enables precise control on their geometry and structure, with efficient material consumption. The bottom-up approach can be implemented with different crystal growth techniques, and two popular options are the molecular beam epitaxy (MBE) and the metal organic vapor phase epitaxy (MOVPE). Both are epitaxial techniques, meaning that the crystal order is not disrupted between the substrate and the NW, and they differ mainly for the supply source of the reactants.

MostII of the NWs used in this thesis were grown using MOVPE, where one or more of the reactants are supplied in vapor phase in the form of metalorganic compounds, which are metals stabilized by organic groups (typically aliphatic compounds, like methyl –CH3 or ethyl –C2H5).

In the MOVPE process, the reactants are supplied in gas phase and then deposited in solid phase, and the growth can be mediated by an intermediate liquid phase, as it will be discussed in the next section. A necessary - but not sufficient - condition to deposit the reactant in solid phase is that the transition from gas to solid is thermodynamically favored, i.e. there is a decrease of the Gibbs free energy which can favor the nucleation of the stable solid phase. In order to grow NW structures, the nucleation and growth need to be done with proper size selection and control, that can be accomplished in different ways. A way to achieve this goal is the particle assisted growth, where a catalytic particle provides a preferential reaction interface for the epitaxial growth, determining also the radial dimension of the NWs. A different approach consists in the particle free growth, where the catalyst is absent:

in this case, the directionality and the size selectivity are typically guaranteed by a mask deposited on a substrate and the process is therefore called selective area epitaxy26-27. It is worth noting that a neat taxonomy does not exist, since many processing variants have been developed. As a matter of fact, the NWs of Paper V have been produced using both a mask and Au particles placed in the openings of

II The NWs of paper I were deposited with a new approach called aerotaxy, which does not require a growth substrate and is treated afterwards.

Figure

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References

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